[Michael C Henry]

I have a Stant radiator pressure tester kit. Not verry expensive. Was the coolant tank full of fluid when cold? are you running with an open or closed circuit cap? I'm running the front surge tank with a closed system cap and it's overflow routed to a 1qt. recovery bottle with hose extending to the bottom of the bottle. I can fill my surge tank when cold because expanded fluid is pushed into recovery bottle and when it cools fluid is pulled back into surgetank keeping cooling system full of fluid. If your expansion tank is a nostalgic open system you need to leave some space in the expansion tank for expanded fluid when warm. With my system I can place the tester inplace of the radiator cap on a full surge tank, start engine and watch as pressure climbs with temperature expansion. If it climbs too fast it may indicate a compression leak into coolant system. At that point I'd remove the tester and watch for bubbles with thew engine running. On your system you may want to fill that expansion tank enough to see the fluid surface from the cap area, Start engine and watch for bubbles in fluid. If you see bubbles that is bad. It should be a moving fluid no bubbles.
OH! another thought, that small hose connecting the intake manifold to tha coolant pump. That is the thermostat bypass. My brother inlaw had a 460( 84 Ford Pickup) that I found had a blister inside that hose, not evident from the outside. That would stop coolant the circulation in the engine so the thermostat wasn't seeing the actual water temperature in the engine. Once the water temp at thermostat was enough the fluid temp in engine was way too high. The thermostat would open violently and the supper heated fluid would be forced out under cap in his closed sysyem and actually blow the fluid out of the coolant recovery tank.. It would make a mess and coolant fluid level was low.